1. Technical Field
The present invention relates generally to circuitry for detecting keypad input and, more specifically, to keypad circuitry operable to identify a pressed key while reducing electromagnetic interference (EMI).
2. Introduction
Conventional keypad circuitry generally comprises a matrix of keys with each key being defined by an intersection of a row and a column, wherein the conventional circuitry performs a key scan to detect and identify a pressed key. An example of such conventional keypad circuitry is disclosed in U.S. Pat. No. 4,918,445, the disclosure of which is hereby incorporated by reference. In the example conventional keypad circuitry, the columns initially function as drivers and the rows initially function as inputs, wherein pull-up circuitry functions to force all row inputs to a logic low value. When a key is pressed, the column driver corresponding to the pressed key is connected to the input of the row corresponding to the pressed key, thereby applying a voltage to the row input and changing the corresponding row input to a logic high value. When the logic high is detected on any of the rows, the columns are changed to inputs and the row corresponding to the pressed key is changed to a driver. The columns, which are now functioning as inputs, are read to detect the column corresponding to the pressed key. When the row and column corresponding to the pressed key are detected, the pressed key is thereby identified.
Operations performed by the conventional keypad circuitry produce a significant amount of electromagnetic interference (EMI). For example, when determining which key is pressed, a signal transition occurs at each of the columns and rows, and the conventional keypad circuitry performs a scan of all the column drivers as well as a scan of all the rows to detect the signal transitions. Additionally, a rail-to-rail voltage swing occurs when the column driver is connected to the input of the row. The rail-to-rail voltage swing and signal transitions of the columns and rows produces EMI, which may cause premature failure or otherwise degrade performance of the keypad circuitry. As such, there exists a need for keypad circuitry capable to identify a pressed key while reducing electromagnetic interference.